1. Field of the Invention
The present invention relates to an intake-and/or exhaust-valve timing control system for internal combustion engines which is variably capable of controlling the intake- and/or exhaust-valve timing depending on the operating state of the engine, for example, the magnitude of engine load and/or engine speed.
2. Description of the Prior Art
Recently, there have been proposed and developed various intake- and/or exhaust-valve timing control systems for internal combustion engines for generating optimal engine performance according to the operating state of the engine.
As is generally known, the valve timing is determined such that optimal engine performance is obtained. However, the predetermined valve timing is not suitable under all operating conditions. For example, when the engine is operating within a range of low revolutions, higher torque will be obtained with an intake-valve timing earlier than the predetermined valve timing.
Such a conventional intake- and/or exhaust-valve timing control system for internal combustion engines has been disclosed in U.S. Pat. No. 4,231,330. In this conventional valve timing control system, a cam sprocket is rotatably supported through a ring gear mechanism by the front end of a cam shaft. The ring gear mechanism includes a ring gear having an inner toothed portion engaging another toothed portion formed on the front end of the camshaft and an outer toothed portion engaging an inner toothed portion formed on the inner peripheral wall of the cam sprocket. In this manner, the ring gear rotatably engages between the cam sprocket and the camshaft. The ring gear is normally biased in the axial direction of the camshaft by spring means, such as a coil spring. At least one of the two meshing pairs of gears is helical. The result is that axial sliding movement of the ring gear relative to the camshaft causes the camshaft to rotate about the cam sprocket and therefore the phase angle between the camshaft and the cam sprocket (that is, the phase angle between the camshaft and the crankshaft) is relatively varied. The ring gear moves as soon as one of the two opposing forces acting on it, namely, the preloading pressure of the above spring means or the oil pressure applied from the oil pump through the flow control valve to the ring gear, exceeds the other. However, in this conventional valve timing control system, each of the two meshing pairs of gears has backlash or play therebetween. During operation of the ring gear, the backlash results in collision between the teeth and thereby causes noise and fluctuations in the torque of the camshaft.
To avoid the above problem, an improved conventional intake- and/or exhaust-valve timing control system has been disclosed in Japanese Patent First Publication (Jikkai Showa) 61-279713. In this valve timing control system, the ring gear, which is disposed between the timing pulley and the camshaft, includes a pair of ring gear elements. One such conventional valve timing control system is shown in FIGS. 1 and 2.
Referring now to FIG. 1, a ring gear member 3 is comprised of a first ring gear element 3c and a second ring gear element 3d. The first and second ring gear elements 3c and 3d are formed in such a manner as to divide a relatively large ring gear including inner and outer toothed portions 3b and 3a into two parts by cutting or milling. Therefore, the first and second ring gear elements 3c and 3d have essentially the same geometry with regard to the inner and outer teeth. These ring gear elements 3c and 3d are interconnected by a plurality of connecting pins 3f which are fixed on the second ring gear element 3d through the annular hollow defined in the first ring gear element 3c. The annular hollow is traditionally filled with elastic materials, such as cylindrical rubber bushing attached by vulcanizing. Alternatively, as shown in FIG. 1, a plurality of coil springs 3e may be provided in the annular hollow, while the springs 3e are supported by the heads of the connecting pins 3f serving as spring seats. In this conventional timing control system, when the first and second ring gear elements 3c and 3d, and the connecting pins 3f are assembled, the first and second ring gear elements 3c and 3d are interconnected in such a manner as to be slightly offset from each other. In other words, the angular phase relationship between the two ring gears 3c and 3d is designed so as to be set to an angular position slightly offset from an angular position in which the tooth traces between the two ring gear elements 3c and 3d are exactly aligned with each other. The above mentioned offset is preset to a slightly greater value than the offset of the ring gear member when meshed with its connecting gears. In this construction of the ring gear member 3, due to the offsetting of the elements 3c and 3d, the apparent tooth thickness of each tooth of the ring gear member is greater than the actual tooth thickness. In FIG. 1, reference numeral 1a denotes an inner toothed portion formed on the inner peripheral wall of a timing pulley 1, while reference numeral 2a denotes an outer toothed portion formed on the outer peripheral wall of a sleeve 2b fixed on the front end of a camshaft 2. Reference numeral 4a denotes an oil pump to generate the working oil pressure used to activate axial sliding movement of the ring gear member. Reference numeral 5 designates a timing belt for transmitting torque from the engine crankshaft to the timing pulley 1. At least one of the two meshing pairs of teeth (1a, 3a, and 2a, 3b) is helical to provide axial sliding movement of the ring gear relative to the camshaft 2. The procedure for installation of the ring gear 3 will be as follows:
First, the outer toothed portion 3a of the ring gear assembly and the inner toothed portion 1a of the pulley 1 are meshed with each other under a condition wherein the two ring gear elements 3c and 3d are twisted relative to each other so as to reduce the previously described apparent tooth thickness, in other words, the teeth of the ring gear elements 3c and 3d are substantially aligned thereby facilitating engagement between the outer toothed portion 3a and the inner toothed portion 1a.
After this, the outer toothed portion 2a of the sleeve 2b is meshed with the inner toothed portion 3b of the ring gear assembly. However, engagement between the outer toothed portion 2a and the inner toothed portion 3b is not facilitated. FIG. 2 shows the positional relationship of the tooth traces of the inner teeth 3b of the first and second ring gear elements 3c and 3d at the pitch circle of the ring gear member 3. As clearly seen in FIG. 2, under a condition wherein the outer toothed portion 3a of the ring gear 3 is engaged with the inner toothed portion 1a of the timing pulley, the apparent tooth thickness t of the inner toothed portion 3b is slightly greater than the actual tooth thickness, since backlash between the outer toothed portion 3a and the inner toothed portion 1a is eliminated by the return spring force generated by the cylindrical rubber bushing or the coil springs 3e serving as a backlash eliminator. Therefore, the apparent tooth spacing s of the inner toothed portion 3b is less than the actual tooth spacing. That is, the apparent tooth spacing s of the inner toothed portion 3b is substantially equal to the tooth thickness of the toothed portion 2a. As a result, the outer toothed portion 2a of the camshaft 2 is easily meshed with the inner toothed portion 3b of the second ring gear element 3d. However, the toothed portion 2a is not easily meshed with the inner toothed portion 3b of the first ring gear element 3c, since a portion of the side wall of the toothed portion 2a tends to abut that of the toothed portion 3b as best seen in FIG. 2. If the machining accuracy of the meshing pair of teeth (3b, 2a) is low, there is a possibility that the apparent tooth spacing s at a particular section of the toothed portion 3b is less than the disirable apparent tooth spacing or that the actual tooth thickness at a particular section of the toothed portion 2a exceeds the disirable tooth thickness. Furthermore, if the machining accuracy is low and for instance the actual backlash between the inner toothed portion 1a of the timing pulley 1 and the outer toothed portion 3a of the first or second ring gear element is greater than a predetermined designed value, the offset between the tooth traces of the two ring gear elements 3c and 3d is increased and as a result the apparent tooth spacing s of the inner toothed portion 3b of the ring gear member is reduced. Under these conditions, engagement between the inner and outer toothed portions 3b and 2a becomes extremely difficult. Therefore, it is desirable that the machining accuracy of the toothed portions 2a and 3b is high and the backlash of the meshing pair of teeth is small. However, this results in an increase of the overall cost of manufacturing the valve timing control system. Moreover, in such conventional valve timing control systems, during the engagement between the inner and outer toothed portions 3b and 2a, assuming that the first ring gear element 3c abuts a hypothetical abutting surface 1b of the timing pulley 1, these toothed portions are forcibly meshed with each other by pressure after the engagement between toothed portions 1a and 3a, thereby resulting in damage to toothed portions 2a and 3b.